ASM320. Distribution. Distribution

Transcription

1 ASM320 Water and Waste Water Operations Distribution Distribution Distribution includes all aspects of getting safe drinking water out into the community Distribution involves attention to: Contamination from cross-connection Backflow and Back-siphonage Leak detection Water towers / tanks Fire hydrants Metering Maintaining quality in the pipes Monitoring Distribution Operator In larger water systems, there may be one or more people whose entire job description relates to operation. There is a separate distribution operator s certificate - distinct from a water treatment plant operator s certificate. 1

2 Cross-connection Potable water drinking water Cross-connection any connection between a potable water system and a source of contamination through which contaminated water could enter the potable water system Cross-connection Backflow Backflow the flow of any water, foreign liquids, gases, or other substances back into a potable water system. Backpressure Backsiphonage Backpressure and back-siphoning Backpressure condition in which the foreign substance is forced into a water system because it is under a higher pressure than the system pressure. Back-siphoning condition in which the water system pressure is less than atmospheric pressure (under vacuum), and the foreign substance is essentially sucked into the potable water system. 2

3 Backpressure and back-siphoning Causes of back-flow Garden hose that s higher than the faucet could push water back into the home Cause of back-siphonage: Flow from water treatment plant is slowed or stopped pressure in lines would be higher Loss of pressure because large volumes of water are diverted elsewhere in the system (e.g. fire fighting) Backpressure and back-siphoning Risks of cross-contamination under backpressure or backsiphoning Chemical dispenser connected to garden hose chemical could go back into house Hose hanging from faucet into kitchen sink if hose is submerged, it could suck dirty water back into the lines Similar setups in canneries, factories, hospitals, or mortuaries (YUK!) Backflow Control Devices Air gaps never let delivery hose go all the way into a tank Reduced-Pressure-Zone Backflow Preventer (PRZ, RPBP, or RPZBP) consists of two spring-loaded check valves with a pressure-regulated relief valve located between them. If there is backsiphonage, both check valves will close, and the space between them is opened to atmospheric pressure. If backpressure, both check valves will close. Check valves permit flow in only 1 direction. PRZ s must be maintained! Air gaps must be enforced! 3

4 Leak Detection Problems with leaky water mains: Wastes water costs money Undermines soil under pavement, can cause pavement to collapse Can cause flooding into basements property damage liability Can cause icing on roads in winter property damage liability Detecting leaks Ask sewer maintenance workers to watch for unusual water in sewer Infrared thermometer gun Look for green patches of grass Leak Detection Detecting Leaks, continued Listening devices (aquaphone) move from hydrant to hydrant and listen for sound of leaking water Do this at night, less traffic noise and less water usage Water audit account for water through all known meters, and if it doesn t add up, you have a leak. Once you have pinpointed an area, install temporary meters to narrow the area Water towers / tanks Purposes of water storage Get water up to a higher elevation to give head (pressure) to the system May balance costs of cost of electricity varies over the day. E.g. pump to water tower at night, gravity feed during the day. Store water at night that people will use in the day (so water treatment plan can run at an average rate all day). Increase chlorine detention time 4

5 Water towers / tanks Other considerations Best place in the system to put the water tower (to add head) Close to water plant Near farthest point in the system Near the lowest pressure Safety considerations of working at water towers Winter operation (keep water surface moving up and down so it doesn t freeze!) Cathodic protection against corrosion (immerse anodes with small currents to they will be sacrificed instead of tank wall) Fire hydrants Hydrant uses Fire fighting Flushing water mains to clear the pipes of rust and sediment Furnishing water to flush sewers Filling tank trucks for rural fire fighting, street washing, tree spraying, etc. Construction uses, such as dust suppression, mixing mortar, etc. Fire hydrants Problems caused by hydrant use Stirs up sediments leading to rusty water complaints Hydrants should be closed SLOWLY to prevent causing a water hammer. If flow of a large volume of water is closed too suddenly, it causes a pressure wave. Can move the hydrant if it is not firmly blocked, or could blow out adjoining home service! 5

6 Metering Meters are the cash registers of a public water system! Meters must be properly selected, installed, maintained and read to obtain the necessary funding for plant and system operation.. Types of Meters Positive displacement meters they monitor all flow. Good for small flows, e.g. homes. Larger water meters: Velocity meters (turbine or propeller) Proportional meters Venturi meters Orifice meters Magnetic meters Ultrasonic Meters Maintaining Quality The water system people are responsible for maintaining quality after the water leaves the treatment plant Chlorine residual Lead and copper in homes 6